1. Field of the Invention
The present invention relates to an apparatus for reading image information. More specifically, the present invention relates to an improvement in level detection of an image information signal.
2. Description of the Prior Art
A typical conventional scheme for reading image information on a card such as a paper sheet for a facsimile apparatus is structured such that the optical density of the image information is converted into an electrical analog signal by means of a photoelectric converter and the electrical analog signal is level detected at a predetermined slide level by means of a comparator, for example, whereby an image information signal of bivalued digital form is obtained wherein the image information is represented by either a logical "one" and "zero", with the logical "one" corresponding to the white and the logical "zero" corresponding to the black.
FIG. 1 shows a schematic diagram of a reading/scanning station of a facsimile apparatus, for example, wherein the image on a card 2 for the facsimile placed on a paper feed apparatus 1 is illuminated by the light beam from a light source 3 and the light beam reflecting therefrom is focused on an image sensor 5 by means of a lens 4, whereby image information on the card 2 is converted into an electrical analog signal.
FIG. 2 shows a graph of the electrical analog signal thus obtained of the image pattern from the card by means of the photoelectric sensor 5 shown in FIG. 1,, wherein the analog signal has been represented such that the white is the high level and the black is the low level. Referring to FIG. 2, a region .alpha. shows a case where the background is the white and the gray partly exists in the background, whereas a region .beta. shows a case where the backgrond is the black in which a dot like white pattern of a very small area exists.
A typical conventional image reading apparatus comprises a comparator adapted for level detecting the input signal at a predetermined slice level A for discrimination of the black and white, which slice level A is selected as shown in FIG. 2 with respect to a peak level V.sub.PEAK. However, in the conventional apparatus, the slice level A was selected to be rather close to the peak level V.sub.PEAK, i.e. in a shallow slice level. Therefore, a disadvantage was encountered in that although discrimination can be made between the white and the gray as in the case of the .alpha. region, such discrimination becomes impossible for the comparator having the slice level A in case of the .beta. region because of the low white level at the center, with the result that the .beta. region is judged entirely as a black area.
The above described disadvantage is caused by insufficient resolution of the lens in view of the fact that there is a limit to the resolution of the lens 4 shown in FIG. 1. Therefore, in case where the background is white and light, the light beam reflected from the background impinges on the black portion of a small area, and in case where the background is black and dark, a portion of the light beam impinging on the white portion of a small area surrounded by the black background is absorbed by the surrounding background.
The problem of insufficient resolution of the lens could be solved to some extent by using a lens having a high resolution. However, normally a lens having a high resolution is very expensive. Thus, it is difficult to provide an image information reading apparatus that is inexpensive.